U.S. Pat. No. 5,083,135, issued 21 Jan. 1992, describes a transparent film antenna for a vehicle window in the shape of a "T", with a horizontally elongate principal element spaced from and parallel to the horizontal upper edge of the window aperture and a vertically elongate impedance matching element extending down the window from the center of the principal element. The fill comprises a transparent, electrically conducting material and is disposed between the sheets of glass in a composite glass windshield. The antenna as described in the patent provides satisfactory performance in the commercial AM and FM broadcasting bands and can be adapted for television and/or telephone bands.
The antenna described above performs well, but its presence in the windshield of a vehicle affects the vehicle's appearance and therefore its commercial acceptance. Although the antenna film is generally transparent to visible light, it can appear to an observer to be different in color from the rest of the windshield, due to light diffraction effects. The horizontal principal element is placed in an upper, dark tinted region of a vehicle window to reduce its visibility; but the vertical impedance matching element projects downward into the lighter tinted or un-tinted region of the window; and some may consider the color difference in this portion of the windshield to be visually objectionable.